Photovoltaic module comprising insulation layer with silane groups

ABSTRACT

The present invention relates to a photovoltaic module comprising a solar cell element and an insulation material laminated to at least one side of the solar cell element wherein the insulation material comprises an olefin copolymer which comprises silane group-containing monomer units, to a process for the production of such a photovoltaic module, and to the use of an olefin copolymer which comprises silane group-containing monomer units for the production of an insulation layer of a photovoltaic module.

This application is a continuation of U.S. patent application Ser. No.12/995,804, filed on Dec. 2, 2010, which is a 371 of PCT/EP2009/004192,filed on Jun. 10, 2009, which claims priority to EP application No.08012423.3, filed on Jul. 9, 2008, the disclosures of which are herebyincorporated by reference in their entirety.

The invention relates to a photovoltaic module comprising a solar cellelement and an insulation material laminated to at least one side of thesolar cell element, to a process for the production of such aphotovoltaic module, and to the use of to an olefin copolymer as aninsulation material for solar cell elements.

Photovoltaic power generation by use of solar cell or photovoltaicmodules is gaining increasing interest due to the unique properties ofthis mode of power generation which does not produce noise, harmfulemissions or polluting gases.

The elements which generate power in those modules through theconversion of light into electrical energy are the solar cell elementswhich comprise semiconducting materials such as silicon, gallium-arsenicand copper-indium-selenium.

As the materials used for solar cell elements are brittle and sensitive,they must be mechanically supported and protected against detrimentalenvironmental influences such as rain, hail, condensation andevaporation of water, dust, wind etc. Furthermore, reliable electricalisolation of the solar cell elements must be achieved.

These functions, which must be maintained throughout the entire lifetimeof a photovoltaic module usually being 20 to 30 years, are provided by alaminate structure of the module comprising a transparent protectivefront cover and a bottom protective substrate, with the solar cellelements being fixed between layers of these protective materials by useof layers of an insulation material.

Usually, the module in addition is protected and supported by analuminium frame.

The insulation material for solar cell elements is sometimes alsoreferred to as “encapsulation”, “embedding”, “adhesive” or “pottant”.

The main functions of the insulation layers in the photovoltaic moduleare to secure safe energy transmission within the solar cell and module.In order to do so it has to provide structural support and positioningfor the solar cell circuit assembly, to physically and electricallyisolate the solar cell elements, i.e. to prevent damage of the elementsby external influences and short-circuiting, to to provide thermalconduction, and, at least for the upper insulation layer which isexposed to sunlight, to achieve and maintain maximum optical couplingbetween the solar cell elements and the incident solar radiation, i.e.to provide and keep maximal transmission for the sunlight to the solarcell elements so as to maximize energy yield.

Today, several types of insulation materials are known, such as peroxidecross-linked ethylene-vinyl acetate (EVA) copolymers (see e.g. EP 1 857500), polystyrene-polybutadiene-polystyrene (SBS) block copolymers (seee.g. U.S. Pat. No. 4,692,557), ionomers (see e.g. U.S. Pat. No.6,320,116), and polyurethanes (see e.g. DE 20 220 444).

The most common insulation material by far is, however, peroxidecross-linked ethylene-vinyl acetate (EVA) copolymers which for use inphotovoltaic modules is extruded as a sheet from an ethylene-vinylacetate (EVA) copolymer composition comprising an organic peroxide as across-linking agent and usually an antioxidant. Cross-linking of theethylene-vinyl acetate copolymer is necessary to provide the insulationlayer with sufficient strength at higher temperatures, because in use,typically, the temperature of the photovoltaic module is between 40 to80° C.

The laminated photovoltaic module is produced in a vacuum laminationprocess. In this process, the components of the module after having beenassembled are put into a vacuum lamination apparatus, in which byapplication of an increased temperature of about 100 to 180° C.,typically 150° C., and increased pressure for a time of from about 15 to30 minutes the laminate is formed under vacuum.

It is a first drawback of the use of peroxide cross-linked EVA asinsulation material that comparatively high temperatures and longlamination times are needed which are caused by the need to decomposethe organic peroxide cross-linking agent in the insulation layer and toeffect and finalize cross-linking thereof. Thus, the production speed ofthe photovoltaic module is comparatively low.

The use of peroxide cross-linked EVA as insulation layer has, however,further drawbacks. It is well known that photovoltaic modules show aperformance loss of 1 to 10% per year, and a significant contribution tothat loss is attributed to the degradation of the cross-linked EVAlayers which may occur as a discoloration of the originally colourless,transparent films (see e.g. the article of A. W. Czanderna and F. J.Pern, “Encapsulation of PV modules using ethylene vinyl Acetatecopolymer as a pottant: A critical review”, Solar Energy Materials andSolar Cells 43 (1996) 101-181). Furthermore, other problems have beenreported such as a delamination at interfaces, penetration of liquidwater, short circuits and arcing, cracking of the solar cell elementsdue to expansion/contraction stresses, cell interconnect failures,charring and melting of solder from hot spots, excessive soiling andweathering.

It is an object of the present invention to provide an insulationmaterial with which the drawbacks of the known technologies, especiallyof the use of peroxide cross-linked EVA copolymers as insulationmaterial, can be avoided. In particular, it is an object of theinvention to provide an insulation material which allows to improve andfacilitate the production process of the photovoltaic module, e.g. byshortening the time necessary for lamination of the module, and, at thesame time, has a lower tendency to degrade and provides an improvedprotection for the solar cell elements from detrimental externalinfluences.

The present invention is based on the finding that the above objects canbe achieved and such an insulation material can be provided if in theinsulation material cross-linkable silane groups are present.

The invention therefore provides a photovoltaic module comprising asolar cell element and an insulation material laminated to at least oneside of the solar cell element wherein the insulation material comprisesan olefin copolymer which comprises silane group-containing monomerunits.

Special advantages of the photovoltaic module of the invention are thatit can be manufactured in a shorter time compared to common modulescomprising to peroxide containing EVA and hence productivity of themodule lamination process is increased. This is due to the fact that,for example, cross-linking of the silane groups takes place already atambient conditions. Accordingly, lower temperatures can be appliedduring lamination, and, as the cross-linking by silanes takes place“automatically” after the lamination process, no residence time isneeded for the cross-linking process during the lamination step.

Corresponding residence time for the EVA insulation materials based on1.47 wt-% of the commonly used peroxide2,5-bis(tert.-butyldioxy)-2,5-dimethylhexane (Lupersol 101) to reach agel content of 70% as a function of temperature are, for example, 45minutes at 140° C., 15 minutes at 150° C., or 6 minutes at 160° C. (Datataken from Paul B Willis, “Investigation of materials and processes forsolar cell encapsulation”, JPL Contract 954527 S/L Project 6072:1,1986).

The reduced lamination temperatures make it possible to use a widerrange of front covers and back-sheets. For example, silane cross-linkingwill open up for replacing glass as front cover by lighter materials,e.g. polycarbonate, as the lamination process easily can take placebelow 130° C. As a load bearing material polycarbonate can combineproperties like high stiffness with low weight and high toughness. Theseproperties will facilitate the construction of roofs, lower breakagewaste and reduce costs of fabrication, transportation and installation.

It is a further advantage that no corrosive and other by-products areformed upon cross-linking of the insulation material which may damagethe photovoltaic cells. The by-products of the peroxide decompositionsare furthermore volatile and in order to avoid void formation during thelamination process, pressure needs to be maintained until the laminatehas been cooled down below the boiling points of the volatiledecomposition products, typically below 80° C.

Still further, prior to lamination a thin film is produced whichtypically is 0.5 mm thick. For peroxide cross-linkable compounds thefilm extrusion process has to be made at much lower extrusiontemperatures than optimal for film extrusion, to reducing the output.Even at low extrusion temperature precuring (scorch) occurs which isseen as an increased gel content of the films. Larger gels result inruptures of the film. The formed gels also negatively affect thelight-transmission and reduce the insulation properties.

Silane cross-linkable compounds do not have these limitations and outputcan be raised due to the possibility to use more typical film extrusiontemperatures, also resulting in a larger freedom in designing theinitial melt viscosity of the polymer. The quality of the film and thephotovoltaic module is raised due to the reduced risk for precuring,especially if proper scorch retardant additives as outlined in e.g. EP449 939 or U.S. Pat. No. 5,350,812 are used.

The life length of a polymeric material is dependent on the stability ofthe polymer as such, but in order to withstand the elevated temperatureand sun-irradiation in a photovoltaic module it is essential to add heatand UV stabilizers. The radicals formed during the peroxidecross-linking process destroy a great portion of these additives. As noradicals are involved in silane curing these kind of interactions willnot occur.

In order to protect the solar cell element from water penetration it isessential to achieve good adhesion between the solar cell element andthe insulation layer. Silane groups have been shown to covalently reactwith polar surfaces like glass, concrete, aluminium etc. Accordingly,the incorporated silane groups have two functions, namely cross-linkingand adhesion promotion.

Preferably, the silane groups present in the insulation material are atleast partly cross-linked.

In this embodiment, the insulation layer not only benefits from theincreased adhesion to the solar cell module due to the presence of thesilane groups, but also from the advantages of the silane groupcross-linking process.

The olefin copolymer which comprises silane group-containing monomerunits preferably is an ethylene or propylene copolymer, more preferablyis an ethylene copolymer.

In a preferred embodiment, the olefin copolymer comprises additionalcomonomer units different from the main olefin monomer units and thesilane group-containing monomer units.

In this embodiment, the olefin copolymer can be provided with improvedis properties such as softness and adhesiveness beneficial for its useas insulation material.

Such additional comonomer units may be chosen from polar and non-polarcomonomers.

For example, non-polar comonomers may be selected from ethylene andalpha-olefins having three or more carbon atoms, preferably having 2 to20 carbon atoms, and more preferably having 3 to 8 carbon atoms.Preferred examples of alpha-olefins are propylene, 1-butene, 1-hexene,1-octene and 4-methyl-1-pentene, and aromatic vinyl compounds, such asstyrene and alpha-ethyl styrene.

However, preferably, the additional comonomer units are polar comonomerunits, so that an olefin copolymer including additional polar monomerunits is used in the insulation layer, mainly in view of the goodtransparency and adhesion to the solar cell elements and otherprotective materials.

Such comonomers include (a) vinyl carboxylate esters, such as vinylacetate and vinyl pivalate, (b) acrylates and alkylacrylates, such asmethylacrylate and methylmethacrylate, ethylacrylate and ethylmethacrylate, butylacrylate and butylmethacrylate, (c) olefinicallyunsaturated carboxylic acids, such as acrylic acid and alkylacrylicacids such as methacrylic acid, maleic acid and fumaric acid, (d)acrylic acid and alkylacrylic acid derivatives, such as acrylonitrileand methacrylonitrile, and acrylic amide and methacrylic amide, (e)vinyl ethers, such to as vinyl methyl ether and vinyl phenyl ether.

It is preferred that the polar comonomer is selected from the group ofC₁- to C₆-alkyl acrylates, C₁- to C₆-alkyl methacrylates, acrylic acid,methacrylic acid and vinyl acetate, and more preferred the polarcomonomer is selected from the group of C₁- to C₄-alkyl, such as methyl,ethyl, propyl or butyl, acrylates or vinyl acetate.

Still more preferably, the polar comonomer is selected from the group ofbutyl acrylate, ethyl acrylate, methyl acrylate, butyl alkylacrylates,ethyl alkylacrylates and methyl alkylacrylates, especially from butylacrylate, ethyl acrylate and methyl acrylate.

A particularly preferred polar comonomer comprises, or consists of,methylacrylate and methylalkylacrylates, especially methylacrylate,because of the low degradation tendency of olefin/methylacrylate andolefin methylalkylacrylates, especially ethylene methyl acrylate (EMA)copolymers.

Preferably, the content of the additional comonomer units in the olefincopolymer is from 2 to 60 wt. %, more preferably is from 5 to 50 wt. %,still more preferably is from 10 to 45 wt. %, and most preferably isfrom 20 to 40 wt. %.

Furthermore, it is preferred that the content of the additionalcomonomer units in the olefin copolymer is from 2 to 30 mol %, morepreferably is from 4 to 25 mol %, still more preferably is from 6 to 20mol %, and most preferably is from 8 to 18 mol %.

The olefin copolymer of the insulation material comprises hydrolysablesilane groups.

The hydrolysable silane groups may be introduced into the polyolefin bycopolymerisation of e.g. ethylene monomers with silane group containingcomonomers, or by grafting, i.e. by chemical modification of the polymerby addition of silane groups mostly in a radical reaction. Bothtechniques are well known in the art, e.g. from U.S. Pat. No. 4,413,066,U.S. Pat. No. 4,297,310, U.S. Pat. No. 4,351,876, U.S. Pat. No.4,397,981, U.S. Pat. No. 4,446,283 and U.S. Pat. No. 4,456,704.

Cross-linking may be performed by condensation of silanol groupscontained in the polyolefin which are obtained by hydrolysation ofsilane groups.

Preferably, the silane group-containing polyolefin has been obtained bycopolymerisation.

The copolymerisation is preferably carried out with an unsaturatedsilane compound represented by the formula

R¹SiR² _(g)Y_(3-q)  (V)

wherein

-   -   R¹ is an ethylenically unsaturated hydrocarbyl, hydrocarbyloxy        or (meth)acryloxy hydrocarbyl group,    -   R² is an aliphatic saturated hydrocarbyl group,    -   Y which may be the same or different, is a hydrolysable organic        group and    -   q is 0, 1 or 2.

Special examples of the unsaturated silane compound are those wherein R¹is vinyl, allyl, isopropenyl, butenyl, cyclohexanyl orgamma-(meth)acryloxy propyl; Y is methoxy, ethoxy, formyloxy, acetoxy,propionyloxy or an alkyl- or arylamino group; and R², if present, is amethyl, ethyl, propyl, decyl or phenyl group.

A preferred unsaturated silane compound is represented by the formula

CH₂═CHSi(OA)₃  (VI)

wherein A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4carbon atoms.

The most preferred compounds are vinyl trimethoxysilane, vinylbismethoxyethoxysilane, vinyl triethoxysilane,gamma-(meth)acryloxypropyl-trimethoxysilane,gamma(meth)acryloxypropyltriethoxysilane, and vinyl triacetoxysilane.

The copolymerisation of the olefin, e.g. ethylene, the unsaturatedsilane compound and, optionally, further monomers may be carried outunder any suitable conditions resulting in the copolymerisation of themonomers.

The amount of the silane group-containing monomer units in the olefincopolymer preferably is from 0.1 to 10 wt. %, more preferably is from0.2 to 8 wt. %, and most preferably is from 0.5 to 5 wt. %.

The olefin copolymer preferably has a MFR₂ (ISO 1133, 2.16 kg/190° C.)of 0.5 to 100 g/10 min, more preferably of 2.5 to 50 g/10 min.

For cross-linking of olefin copolymers comprising monomer units withsilane groups, a silanol condensation catalyst is preferably used, sothat in the photovoltaic module of the invention cross-linking of thesilane groups of the olefin copolymer preferably has been effected bythe use of a silanol condensation catalyst.

Thus, preferably the composition used for the production of theinsulation layer of the photovoltaic module of the invention comprises asilanol condensation catalyst.

Conventional catalysts are for example tin-organic compounds such asdibutyl tin dilaurate (DBTDL).

It is further known that the cross-linking process advantageously iscarried out in the presence of acidic silanol condensation catalysts. Incontrast to the conventional tin-organic catalysts, the acidic catalystsallow cross-linking to quickly take place already at room temperature.Such acidic silanol condensation to catalysts are disclosed for examplein WO 95/17463. The contents of this document is enclosed herein byreference.

The silanol condensation catalysts of the polyolefin compositionfurthermore preferably is a Brönsted acid, i.e. is a substance whichacts as a proton donor.

The Brönsted acids may comprise inorganic acids such as sulphuric acidand hydrochloric acid, and organic acids such as citric acid, stearicacid, acetic acid, sulphonic acid and alkanoic acids as dodecanoic acid,or a precursor of any of the compounds mentioned.

Preferably, the Brönsted acid is a sulphonic acid, more preferably anorganic sulphonic acid.

Still more preferably, the Brönsted acid is an organic sulphonic acidcomprising 10 C-atoms or more, more preferably 12 C-atoms or more, andmost preferably 14 C-atoms or more, the sulphonic acid furthercomprising at least one aromatic group which may e.g. be a benzene,naphthalene, phenantrene or anthracene group. In the organic sulphonicacid, one, two or more sulphonic acid groups may be present, and thesulphonic acid group(s) may either be attached to a non-aromatic, orpreferably to an aromatic group, of the organic sulphonic acid.

Further preferred, the aromatic organic sulphonic acid comprises thestructural element:

Ar(SO₃H)_(x)  (II)

with Ar being an aryl group which may be substituted or non-substituted,and x being at least 1.

The organic aromatic sulphonic acid silanol condensation catalyst maycomprise the structural unit according to formula (II) one or severaltimes, e.g. two or three times. For example, two structural unitsaccording to formula (II) may be linked to each other via a bridginggroup such as an alkylene group.

Preferably, Ar is a aryl group which is substituted with at least oneC₄- to C₃₀-hydrocarbyl group, more preferably C₄- to C₃₀-alkyl group.

Aryl group Ar preferably is a phenyl group, a naphthalene group or anaromatic group comprising three fused rings such as phenantrene andanthracene.

Preferably, in formula (II) x is 1, 2 or 3, and more preferably x is 1or 2.

Furthermore, preferably the compound used as organic aromatic sulphonicacid silanol condensation catalyst has from 10 to 200 C-atoms, morepreferably from 14 to 100 C-atoms.

In one preferred embodiment, Ar is a hydrocarbyl substituted aryl groupand the total compound containing 14 to 28 carbon atoms, and stillfurther preferred, the Ar group is a hydrocarbyl substituted benzene ornaphthalene ring, the hydrocarbyl radical or radicals containing 8 to 20carbon atoms in the benzene case and 4 to 18 atoms in the naphthalenecase.

It is further preferred that the hydrocarbyl radical is an alkylsubstituent having 10 to 18 carbon atoms and still more preferred thatthe alkyl substituent contains 12 carbon atoms and is selected fromdodecyl and tetrapropyl. Due to commercial availability it is mostpreferred that the aryl group is a benzene substituted group with analkyl substituent containing 12 carbon atoms.

The currently most preferred compounds are dodecyl benzene sulphonicacid and tetrapropyl benzene sulphonic acid.

The silanol condensation catalyst may also be precursor of the sulphonicacid compound, including all its preferred embodiments mentioned, i.e. acompound that is converted by hydrolysis to such a compound. Such aprecursor is for example the acid anhydride of a sulphonic acidcompound, or a sulphonic acid that has been provided with a hydrolysableprotective group, as e.g. an acetyl group, which can be removed byhydrolysis.

In a second preferred embodiment, the sulphonic acid catalyst isselected from to those as described in EP 1 309 631 and EP 1 309 632,namely

a) a compound selected from the group of(i) an alkylated naphthalene monosulfonic acid substituted with 1 to 4alkyl groups wherein each alkyl group is a linear or branched alkyl with5 to 40 carbons with each alkyl group being the same or different andwherein the total number of carbons in the alkyl groups is in the rangeof 20 to 80 carbons;(ii) an arylalkyl sulfonic acid wherein the aryl is phenyl or naphthyland is substituted with 1 to 4 alkyl groups wherein each alkyl group isa linear or branched alkyl with 5 to 40 carbons with each alkyl groupbeing the same or different and wherein the total number of carbons inthe alkyl groups is in the range of 12 to 80;(iii) a derivative of (i) or (ii) selected from the group consisting ofan anhydride, an ester, an acetylate, an epoxy blocked ester and anamine salt thereof which is hydrolysable to the corresponding alkylnaphthalene monosulfonic acid or the arylalkyl sulfonic acid;(iv) a metal salt of (i) or (ii) wherein the metal ion is selected fromthe group consisting of copper, aluminium, tin and zinc; andb) a compound selected from the group of(i) an alkylated aryl disulfonic acid selected from the group consistingof the structure (Ill):

and the structure (IV):

wherein each of R₁ and R₂ is the same or different and is a linear orbranched alkyl group with 6 to 16 carbons, y is 0 to 3, z is 0 to 3 withthe proviso that y+z is 1 to 4, n is 0 to 3, X is a divalent moietyselected from the group consisting of —C(R₃)(R₄)—, wherein each of R₃and R₄ is H or independently a linear or branched alkyl group of 1 to 4carbons and n is 1; —C(═O)—, wherein n is 1; —S—, wherein n is 1 to 3and —S(O)₂—, wherein n is 1; and(ii) a derivative of (i) selected from the group consisting of theanhydrides, esters, epoxy blocked sulfonic acid esters, acetylates, andamine salts thereof which is a hydrolysable to the alkylated aryldisulfonic acid,together with all preferred embodiments of those sulphonic acids asdescribed in the mentioned European Patents.

Preferably, in the polyolefin composition used for the production of theinsulation material of the photovoltaic module, the silanol condensationcatalyst is present in an amount of 0.0001 to 6 wt. %, more preferablyof 0.001 to 2 wt. %, and most preferably 0.02 to 0.5 wt. %.

Preferably, in the final insulation material the olefin copolymer iscross-linked to a degree of at least 50%, more preferably at least 60%,and most preferably to at least 70%.

The final insulation material preferably has a total hemispherical lighttransmission larger than 90% of incident light at 400-1100 nm.

Preferably, the final insulation material has a glass transitiontemperature below −40° C.

Furthermore, preferably the final insulation material has a waterabsorption of <0.5 wt-% at 20° C./100% relative humidity.

Still further, the final insulation material preferably has a tensilemodulus of 100 MPa or less, more preferably of 50 MPa or less, stillmore preferably of 30 MPa or less and most preferably of 25 MPa or less(ISO 527-2, 1 mm/min).

Finally, the final insulation material preferably has a brittlenesstemperature of −40° C. or below.

In the photovoltaic module of the invention, the insulation layer(s)preferably has a thickness of 0.1 to 5 mm, more preferably of 0.2 to 3mm, and most preferably of 0.3 to 1 mm.

In the composition used for the production of the insulation layer, inaddition to the olefin copolymer comprising silane-group containingmonomer units and the silanol condensation catalyst further componentsmay be present.

However, it is preferred that the olefin copolymer comprisingsilane-group containing monomer units is present in the composition formaking the insulation layer in an amount of at least 30 wt. %, morepreferably at least 50 wt. % and most preferably at least 90 wt. %.

Preferred additives present in the composition used for the productionof the isolation layer are UV absorbers, UV stabilisers, and/oranti-oxidants.

Furthermore, preferably a scorch retarding agent is present in thecomposition.

The amount of additives, preferably of UV absorbers, UV stabilisers,anti-oxidants and/or scorch retarding agent, which are present in thecomposition for making the insulation layer, preferably is 5 wt. % orless, more preferably of 3 wt. % or less.

The photovoltaic module of the invention furthermore preferablycomprises a transparent protective front cover. Any common type of frontcover can be used, such as made of glass, acrylic resins, polycarbonateresins, polyester resins, and fluorine-containing polymers.

Typically, the thickness of the front cover is from 1 to 10 mm, morepreferably from 2 to 6 mm.

Insulation layer(s) comprising an olefin copolymer which comprisessilane-group containing monomer units in any of the above describedembodiments are well suited for use together with polycarbonate frontcovers, because a lower temperature can be used in the laminationprocess for producing the photovoltaic module laminate. Thus, in oneembodiment of the photovoltaic module of the invention, the modulecomprises a front cover comprising or consisting of a polycarbonateresin.

The photovoltaic module of the invention furthermore preferablycomprises a protective back or bottom cover. Any type of back cover canbe used, such as a metal or a single- or multi-layer sheet such as afilm of a thermoplastic resin. Examples of the back cover protectivematerial include metals such as tin, aluminum and stainless steel,inorganic materials such as glass and single- or multi-layer protectivematerials of polyester, inorganic material-metallized polyesters,fluorine-containing resins and polyolefins. A commonly used back coveris a three layer laminate made of a first polyvinyl fluoride (PVF)layer, a water vapour barrier layer such as a metal e.g. aluminium layeror a polyethylene terephthalate layer, and a second polyvinyl fluoride(PVF) layer. Such a three layer film comprising polyethyleneterephthalate as intermediate layer is also denoted as TPT film.

The possibilities of a reduced lamination temperature of the inventionopens up for a wider use lighter, less hydrolyse sensitive, and cheapermaterials e.g. high density polyethylene and polypropylene which alsohave excellent electrical and barrier properties. Thus, in oneembodiment the photovoltaic module of the invention a sheet or film as aprotective back or bottom cover which comprises polyethylene and/orpolypropylene.

Usually, the thickness of the back cover is from 50 to 500 micrometer,more to preferably from 80 to 250 micrometer.

Typically, the photovoltaic module of the invention comprises, orconsists of, a front cover, a first insulation layer, solar cellelements which are interconnected by a conducting material, a secondinsulation layer, and a back cover, optionally supported by a metal suchas aluminium frame.

In the photovoltaic module of the invention, at least one, butpreferably both, insulation layer(s) comprise an olefin copolymer whichcomprises silane group-containing monomer units in any of theabove-described embodiments.

The present invention is also directed to a process for the productionof a photovoltaic module which comprises laminating a sheet of aninsulation material comprising an olefin copolymer which comprisessilane group-containing monomer units in any of the above describedembodiments to a solar cell element.

Usually, the composition used for the production of the insulation layeris extruded into a sheet having the desired thickness.

It is preferred that during extrusion cross-linking is avoided as far aspossible, i.e. that no scorch occurs. This may be provided by the use ofa scorch retarding agent, such as described e.g. in EP 449 939 or U.S.Pat. No. 5,350,812.

Cross-linking of the extruded sheet may be partially or completelycarried out before the lamination step for producing the finalphotovoltaic module laminate.

However, the extruded sheet may also be cross-linked to the desireddegree only during or after the lamination process.

The lamination process can be carried out in a conventional laminatorapparatus.

Preferably, the temperature during the lamination process is 150° C. orbelow, more preferably is 130° C. or below, and most preferably is 100°C. or below.

The pressure used during lamination usually is below 2 bar, morepreferably is below 1 bar.

The total lamination time preferably is below 30 min, more preferably isbelow 20 min, and most preferably is below 10 min.

The invention furthermore relates to the use of an olefin copolymerwhich comprises silane group-containing monomer units in any of theabove described embodiments as an insulation material for solar cellelements.

The present invention will be further illustrated by way of an example,and by reference to the following figures:

FIG. 1: Results of peeling tests showing influence of vinyl trimethoxygroups on the adhesion strengths towards aluminium.

FIG. 2: Results of peeling tests after ageing in water showing increaseof adhesion strength after treatment in water.

A sheet useable as an insulation layer of a photovoltaic module wasproduced starting from a composition with the components as given inTable 1.

Three ethylene terpolymers (Terpolymers 1, 2 and 3) consisting of vinyltrimethoxysilane silane (VTMS) and methylacrylate (MA) or butylacrylate(BA), respectively, were produced on commercial tubular reactors at 2800to 3300 bar and 240 to 280° C. peak temperatures.

The polymers are described in Table 1 and compared with the EVA polymerElvax 150 (Comparative Example 1), which has been the dominatingmaterial for insulation following the recommendations from the “Low costflat plate silicon array project” carried out at the Jet PropulsionLaboratory. The project started 1975 and the outcome of the project isreported in Paul B Willis, “Investigation of materials and processes forsolar cell encapsulation”, JPL Contract 954527 S/L Project 6072:1, 1986(hereinafter denoted as ref JPL).

TABLE 1 Comparative Example Test (Elvax method Terpolymer 1 Terpolymer 2Terpolymer 3 150) Monomer Ethylene Ethylene Ethylene Ethylene Comonomer1 Type MA MA BA VA Amount, wt-% 31 20 17 32 Amount, mol-% 12.7 7.5 4.213.2 Comonomer 2 Type VTMS VTMS VTMS None Amount, wt-% 1.0 1.2 1.9 0MFR_(2,16), ISO 3.0 10.1 5.3 35 g/10 min (2.16 kg/ 1133 190° C.) DensityISO 958 928 957 2781 Melt ISO 70 76 96 63 temperature, ° C. 3146 Vicatsoftening ISO 306 <40 <40 36 point at 10N, ° C. Hardness ISO 868 53 8865-73 Shore A Hardness ISO 868 10 28 24 Shore D Tensile ISO 2.4 20 312.2 modulus, MPa 527-2, 1 mm/min Tensile strength ISO 37, 8 13 7.5 atbreak, MPa 50 mm/min Brittleness ISO 812 <−70 <−70 −100 temperature, °C. Volume IEC 93 1.4 × 10¹⁴ 2.5 × 10¹⁶ 1 × 10¹⁴ resistivity, Ω-cm at 20°C. Dielectric IEC 250 3.21 2.69 2.6 constant at 50 Hz, 20° C. Powerfactor at IEC 250 0.002 0.0009 0.003 50 Hz,

In Table 2 a summary of the key properties of insulation materials forPV-modules are collected based on the finding in the JPL report (refJPL).

TABLE 2 Characteristic Requirement Source Glass transition temperature<−40° C. (1) Total hemispherical light >90 incident (1) transmission at400-1100 nm Hydrolysis None at 80° C., 100% (1) relative humidityResistance to thermal oxidation Stable up to 85° C. Mechanical creepNone at 90° C. Tensile modulus <20.7 MPa UV absorption degradation Noneat wavelength >350 nm Hazing or clouding None at 80° C., 100 relativehumidity Odor, human hazards None VA content >25% in order to reach anacceptable optical transmission of 89 to 92% Gel content >70% results inresistance in thermal creep with margin at 110° C.

Table 1 shows that the incorporation of VTMS into the polymer chain andthe replacement of vinylacetate by butylacrylate do not result in anysignificant differences in tensile strength, tensile modulus, electricaland low temperature properties as well as in hardness and melting point.It is also shown that these properties these can be controlled by themolar amount of comonomer 2. As to outlined in ref JPL, the opticalproperties and tensile modulus are controlled by the crystallinity ofthe polymer and in order to reach an acceptable optical transmission theVA content should be >25 wt-% (>9.8 mol-%).

In order to control the cross-linking properties, a tape of a thicknessof 0.5 mm was produced on a Brabender tape extruder with alength/diameter ratio of 20. The temperature setting was 120-150−170° C.Prior to extrusion the different terpolymers were mixed with 5% of amasterbatch consisting of 96.5 wt-% of an ethylene butylacrylatecopolymer (MFR₂=7 g/10 min, BA content=17 wt-%), 1.5 wt-% ofdodecylbenzene sulphonic acid as cross-linking catalyst and 2 wt-%4,4-thiobis(2-tert.butyl-5-methylphenol) as stabilizer. The tapes werestored for 24 hours and 50% relative humidity at 23° C. prior todetermination of cross-linking properties. The gel content wasdetermined by putting milled tapes in boiling decaline for 10 hours.

TABLE 3 Test Ter- Ter- Ter- method polymer 1 polymer 2 polymer 3Gel-content, wt-% Decaline 78 77 82 Hot-set test IEC 811-2-1 200° C.,0.20 MPa Elongation 20 25 15 Permanent set 0 0 0

It can be concluded that all tapes based on any of Terpolymers 1 to 3give suitable cross-linking degrees as insulation for photovoltaicmodules after storage at ambient conditions.

In order to evaluate the cross-linked tapes resistance to hydrolysis,the cross-linked tapes of Terpolymers 1 and 3 were stored in water.After the exposure, tensile strength at break, elongation at breakaccording to ISO 37 (tensile testing speed=500 mm/min) and weightchanges were evaluated. The results are reported in Table 4.

TABLE 4 Weight Tensile Temp., Time, Change, strength Sample Solution °C. Days wt - % MPa Elongation, % Terpol. 1 None — — — 3.0 210 Terpol. 1Water 23 30 0.1 3.0 184 Terpol. 1 Water 50 30 0.1 3.6 234 Terpol. 1Water 100 7 0.7 2.9 166 Terpol. 3 None — — — 4.6 42 Terpol. 3 Water 2330 0.1 5.5 71 Terpol. 3 Water 50 30 0 4.4 41 Terpol. 3 Water 100 7 0.44.9 55

The results show that Terpolymers 1 and 3 are resistant to hydrolysis inconditions relevant for photovoltaic modules.

In order to evaluate the thermal stability of different possiblecopolymers, the following samples were heat treated in athermogravimetic analyzer at 333° C. in a nitrogen atmosphere:

-   EBA-4,3: Ethylene butyl acrylate (BA) copolymer, BA content 17 wt-%    (4.3 mol-%), MFR₂=6 g/10 min,-   EEA-4,8: Ethylene ethylacrylate (EA) copolymer, EA content 15 wt-%    (4.8 mol-%), MFR₂=8 g/10 min,-   EHEMA-1,8: Ethylene hydroxyl ethyl methacrylate (HEMA) copolymer,    HEMA content 8 wt-% (1.8 mol-%), MFR₂=1.5 g/10 min,-   EMA-5,7: Ethylene methyl acrylate copolymer, MA content 15.6 wt-%    (4.8 mol-%), MFR₂=15 g/10 min,-   EMMA-4,9: Ethylene methyl methacrylate (MMA) copolymer, MMA content    14.1 wt-% (4.9 mol-%), MFR₂=8 g/10 min,-   EVA-6,7: Ethylene vinyl acetate (VA) copolymer, VA content 28 wt-%    (6.7 mol-%), MFR₂=8 g/10 min.

The results of the thermal stability tests are shown in Table 5.

TABLE 5 Sample Time at 333° C., min Weight loss, wt-% EBA-4,3 120 4.5EEA-4,8 120 3.8 EHEMA-1,8 120 4.4 EMMA-4,9 120 2.7 EMA-5,7 120 2.6EVA-6,7 90 13.1

It is evident that the most preferred groups for crystallinity controlof terpolymers intended photovoltaic module are methyl acrylate ormethyl methacrylate.

In order to study the influence of the vinyl trimethoxy silane groups onthe polymers adhesion to polar substrates, 200 μm thick films ofTerpolymer 3 and an ethylene vinyl trimethoxy silane copolymercontaining 2 wt-% VTMS, MFR₂=0.9 g/10 min were pressed by putting eachfilm in between 150 μm thick to aluminium foils and pressing together at250° C. for 10 seconds at a pressure of 1.3 MPa.

After one week at ambient conditions, the peel force was tested in anInstron 1122 by a 180° T-peel test, with a crosshead speed of 200mm/min. The width of the test strip were 25 mm. The same test procedurewere also performed on ethylene butylacrylate copolymers and ethyleneacrylic acid copolymers. The latter is known to give excellent adhesionstrength towards polar substrates. The results of the tests are shown inFIG. 1.

The silane group-containing laminates were furtheron treated in waterheated to 85° C. for 37 hours and compared with a laminate in which theplastic film was corona treated low density polyethylene (LDPE). Theresults of the tests are shown in FIG. 2. The tests show that vinyltrimethoxy silane groups have a dramatic influence on the adhesionstrength which even increases after treatment in water.

1. A photovoltaic module comprising a solar cell element and aninsulation material laminated to at least one side of the solar cellelement wherein the insulation material comprises an olefin copolymerobtained by copolymerization which comprises from 0.5 to 5 wt. % ofsilane group-containing monomer units, main olefin monomer units, andfrom 10 to 45 wt % of polar comonomer units, the polar comonomer unitsbeing selected from acrylates and alkylacrylates.
 2. Photovoltaic moduleaccording to claim 1, wherein the silane groups to are at least partlycrosslinked.
 3. Photovoltaic module according to claim 1, wherein theolefin copolymer is an ethylene copolymer.
 4. Photovoltaic moduleaccording to claim 1, wherein the insulation material further comprisesa silanol condensation catalyst.
 5. Photovoltaic module according toclaim 4, wherein the silanol condensation catalyst is a sulphonic acid.6. Photovoltaic module according to claim 1 wherein the amount of thepolar comonomer units in the olefin copolymer is from 20 to 40 wt. %. 7.The photovoltaic module according to claim 1, further comprising aprotective back cover consisting of glass, metal, or a single-layersheet of a thermoplastic resin.
 8. The photovoltaic module according toclaim 7, wherein the protective back cover consists of glass.
 9. Thephotovoltaic module according to claim 7, wherein the protective backcover consists of metal.
 10. The photovoltaic module according to claim7, wherein the protective back cover consists of the single-layer sheet.11. The photovoltaic module according to claim 1, wherein the olefincopolymer does not comprise a peroxide.
 12. Process for the productionof a photovoltaic module which comprises laminating a sheet of aninsulation material comprising an olefin copolymer which comprisessilane group-containing monomer units to a solar cell element.
 13. Aphotovoltaic module comprising a solar cell element and an insulationmaterial laminated to at least one side of the solar cell elementwherein the insulation material comprises an olefin copolymer whichcomprises silane group-containing monomer units.
 14. Photovoltaic moduleaccording to claim 13, wherein the olefin copolymer comprises additionalcomonomer units different from the main olefin monomer units and thesilane group-containing monomer units.
 15. Photovoltaic module accordingto claim 14, wherein the additional comonomer units are polar comonomerunits.
 16. Photovoltaic module according to claim 15, wherein the polarcomonomer units are selected from vinyl carboxylate esters, acrylatesand alkylacrylates, olefinically unsaturated carboxylic acids, acrylicacid and alkylacrylic acid derivatives, and vinyl ethers. 17.Photovoltaic module according to claim 16, wherein the polar comonomerunits are selected from C₁- to C₆-alkyl acrylates, C₁- to C₆-alkylmethacrylates, acrylic acid, methacrylic acid and vinyl acetate. 18.Photovoltaic module according to claim 13, wherein the amount of thesilane of the silane group-containing monomer units in the olefincopolymer is from 0.1 to 10 wt. %.
 19. Photovoltaic module according toclaim 14, wherein the amount of additional comonomer units in the olefincopolymer is from 5 to 60 wt. %.
 20. Photovoltaic module according toclaim 13, wherein the olefin copolymer is an ethylene copolymer.